Localized dehydration of antigorite during experimental deformation at subduction zone conditions

Dehydration embrittlement is the dominant mechanism proposed to explain deep-focus earthquakes between 100–350 km in depth. Antigorite dehydration was extensively investigated in previous experimental studies, which demonstrated contrasting results regarding the seismic potential of antigorite dehydration. Additionally, microstructural aspects of antigorite dehydration and their implications for deep seismicity are scarce. Localized dehydration, on the other hand, might generate strain weakening, potentially leading to failure at depths relevant to deep earthquakes. Localized antigorite dehydration is demonstrated to occur in nature and the laboratory; however, it is not clear if this is a passive or dynamic process.

To better understand the micro-mechanisms of localized antigorite dehydration, we conducted high-pressure, high-temperature experiments under isostatic and non-isostatic conditions. Experiments were run at 3 GPa and temperatures within and above the antigorite stability field (530 °C–800 °C). Antigorite cores with 2 mm diameter were mounted in cubic assemblies and deformed in a 6-ram multi-anvil press at the Bayerisches Geoinstitute. Pure shear deformation was applied by inserting one pair of anvils while simultaneously removing the remaining two pairs orthogonal to it.

Results show that isostatic dehydration of antigorite at 3 GPa starts at ~530 °C and completes at ~800 °C. Localized dehydration occurs in isostatic and non-isostatic conditions within the antigorite stability field. It is enhanced during deformation experiments, resulting in the formation of nanocrystalline veins and networks containing olivine and pyroxene. These results demonstrate that localized dehydration might occur through passive and dynamic processes with the development of different microstructures.